It is well known in the art to produce polymers from monomers such as butadiene and styrene by contacting these monomers in a diluent with an alkali metal initiator. Alkyllithium initiators are oftentimes used and produce polymers with a variety of properties depending upon the specific monomers used and the process steps, as well as the polymerization conditions. At the end of the polymerization, a polymer that is terminated by one or more alkali metal atoms is obtained.
It is also known in the art to couple such alkali metal-terminated polymers, e.g., lithium-terminated polymers. In accordance with this known process, an alkali metal-terminated polymer is treated with a compound having two or more functional groups containing two or more reactive sites capable of reacting with the carbon-alkali metal bonds on the alkali metal-terminated polymer. The multifunctional coupling agent thereby becomes a nucleus for the resulting structure. From this nucleus long-chain polymer branches radiate, and such coupled polymers have specific properties that render them useful for particular applications.
Coupled polymers that are derived from coupling agents possessing three or more reactive sites are frequently called radial polymers in order to distinguish them from linear polymers that are either not coupled at all or that are coupled by only a bifunctional coupling agent. Such radial polymers have been of particular interest because of their increased Mooney viscosity, the improved processability, and the reduced cold flow as compared to the uncoupled or parent polymers. Hydrogenation of the alkadiene-based radial polymers has received considerable attention in recent years because the hydrogenation of such polymers increases the resistance thereof to environmental attack, e.g., to oxygen and/or ozone deterioration, as well as to deterioration by heat, light and particularly ultraviolet light.
It has, however, been found that some of these radial polymers are not sufficiently stable under hydrogenation conditions. More specifically it has been found that radial polymers that have a connection between an inorganic atom and the first carbon atom of the parent polymer chain sometimes are not as stable as radial polymers in which the connection between the coupling agent and the polymer is a carbon-carbon bond. A cleavage of the bond between the inorganic atom of the coupling agent and the carbon atom of the polymer chain occurs under hydrogenation conditions. If this cleavage occurs to a considerable degree, the hydrogenated radial polymer is admixed with hydrogenated but no longer coupled polymer chains which results in impairing the properties of the coupled polymer. Thus it would be desirable to have a coupled polymer available that can be hydrogenated without substantial cleavage of the polymer chains from the coupling agent. Furthermore, it would be desirable to have a coupling agent available which does not contain inorganic atoms connected to the active sites of the coupling agent and susceptible to this cleavage. At the same time it would be desirable that this coupling agent achieves a high degree of coupling efficiency.